funcutils: forward defaulted args as keyword args in wraps invocation

[gaoflow]

Fixes #343.

The bug

FunctionBuilder.get_invocation_str() passes every regular argument
positionally in the generated body:

# generated for:  def pow(x, y, msg=''):
return _call(x, y, msg)

When a caller supplies a defaulted parameter by keyword (pow(3, 2, msg='abc')),
the generated function correctly binds msg='abc', but then forwards it
positionally as _call(3, 2, 'abc'). Any wrapper that inspects
*args / **kwargs separately—the standard pattern—loses the keyword
information:

def flip(f):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return f(*reversed(args), **kwargs)
    return wrapper

def power(x, y, msg=''):
    return (x, y, msg)

flip(power)(3, 2, msg='abc')
# before: ('abc', 2, 3)  – msg landed in args, was reversed
# after:  (2, 3, 'abc')  – correct

The fix

Args that carry a default are now forwarded with the name=name form:

return _call(x, y, msg=msg)

This means the inner wrapper receives them in **kwargs, preserving
exactly how the caller passed them.

As a welcome side-effect, this also lifts the limitation documented in
test_wraps_inner_kwarg_only (issue #261): wrapping a keyword-only-b
function with a signature that exposes b as positional-or-keyword now
works, because the generated invocation passes b=b.

Test changes

• Added test_wraps_preserves_kwargs_for_defaulted_args in
  test_funcutils.py directly covering the issue wraps loses keywords #343 scenario.
• test_wraps_inner_kwarg_only: updated docstring and assertion to
  reflect that wraps(g)(f)(3) now succeeds (was asserting TypeError
  which documented the old limitation).
• test_wraps_expected (expect_pair / expect_dict): these wrappers
  extracted defaulted injected args from args[-1], relying on the old
  positional-passthrough. Updated to use kwargs.pop(...), which is the
  correct pattern under the new semantics.
• Parametrized test_get_invocation_sig_str: updated expected invocation
  string for the single-defaulted-arg case ("a" → "a=a").

All 438 tests pass.